1. Field of the Invention
This invention relates to a real image mode finder in which an object image formed by an objective optical system is observed through an eyepiece optical system, used in a photographic camera, a still video camera or the like, and also relates to a camera using such a real image mode finder.
2. Description of Related Art
In a photographic or video camera, a real image mode finder, in which an object image formed by an objective optical system is observed through an eyepiece optical system, has been widely used. In particular, as an image erecting means provided in this finder, a Porro prism or a roof pentaprism is often adopted.
Recently, the needs for a high variable magnification ratio of the finder and compactness of the camera have been emphasized. In addition to conventional systems, several proposals have been made with respect to the image erecting means of real image mode finders so that finder units should be compact and achieve high magnification.
One of these, for example, Japanese Patent Preliminary Publication No. Hei 8-129203, discloses a real image mode finder. This finder is designed so that a light beam emerging from an objective lens, after being obliquely bent by planar reflecting members toward the pupil side and then toward the object side, is further bent by a roof reflecting surface toward the pupil side and is introduced into an eyepiece. The arrangement of this finder is made so that the planar reflecting member, which obliquely reflects the light beam toward the object side, has a normal line inclined toward the optical axis of the objective lens, thereby enabling the entire length of the finder to be reduced.
On the other hand, Japanese patent Preliminary Publication No. Hei 8-179400 proposes the image erecting means of a real image mode finder. The image erecting means disclosed in this publication is such that, by combining a roof prism and triangular prisms, which are arranged in that order from the object side, a total reflection-transmission property of the surface of one of the triangular prisms is utilized for twice-reflecting a light beam in these prisms. With this image erecting means, an optical path length from an intermediate imaging plane to an eyepiece can be reduced, and thus it becomes possible that the focal length of the eyepiece is reduced to increase the finder magnification.
However, the finder disclosed in Hei 8-129203 mentioned above has the drawback that since it is constructed so that the light beam is reflected once at an angle toward the pupil side by the reflecting surface placed after the objective lens, looking from the object side, the objective lens projects toward the object side and as a result, the finder enlarges along the optical axis. Furthermore, because the light beam passing through the objective lens must be reflected at a certain angle toward the object side, the finder tends to be naturally enlarged in a direction perpendicular to the optical axis of the objective lens. Hence, in the finder, if an attempt is made to prevent the objective lens from projecting and to enlarge an effective beam diameter, the optical path length on the eyepiece side must be increased. This situation is unsuitable for the finder placed in a limited space.
The finder set forth in Hei 8-179400, by contrast, diminishes the size of the eyepiece system. Since, however, a roof surface is used for the reflecting surface placed after the objective lens, viewed from the object side, the angle of inclination made by the roof surface with the optical axis of the objective lens, in contrast with that of a roof-pentaprism type, is liable to increase because of the property of the prism placed after the roof surface. Specifically, arrangements are made so that, in the case of the roof-pentaprism type, its roof surface facilitates the reflection of the light beam toward the object side, while in the finder set forth in Hei 8-179400, its roof surface facilitates the reflection of the light beam toward the pupil side. Hence, the arrangement such that the light beam is reflected toward the object side is lacking in compactness, and this is unfavorable. That is, this finder, compared with the roof-pentaprism type, has the disadvantage that the objective lens is liable to project toward the object side.